Is the superposition principle in quantum mechanics always valid? Is the superposition principle in quantum mechanics always valid, or might there be a reason for it to break down at some high energy scales? What would be the effect of this, and how would one measure it?
 A: Our current theory of (orthodox) quantum mechanics maintains that the superposition principle is valid at all energy scales.
It may be possible that the superposition principle breaks down at high energies. I'm not sure the highest energies to which we've tested the superposition principle.
Some candidates that come to mind:

*

*traditional atomic physics experiments are typically performed around room temperature or colder. Maybe a little hotter but not an order of magnitude hotter.

*We have some facilities which generate x-rays that are used for various imaging processes. I'm not sure if single photon experiments have been performed using these beams or not. For example a two-slit experiment. Such an experiment would be a higher energy test of the superposition principle.

*Experiments which utilize high energy plasmas may rely in some way on the quantum superposition principle. These get quite hot and could be a high energy test of the superposition principle.

*High energy particle experiments rely in some way on the superposition principle in that they make predictions based on particles being in superpositions of states. For example nuetrino oscillation experiments rely on neutrinos being in superpositions of flavors. Sometimes these neutrinos come from the sun so they may be high energy.

*I've heard it hypothesized that neutron stars might be Bose-Einstein Condensates. The physics of neutron stars may then be a test of the superposition principle at high energy.

The most straightforward way to directly test for this would be to perform the double slit experiment with higher and higher energy particles. orthodox quantum mechanics would predict you always see an interference pattern form. If you ever saw that a pattern didn't form, (and there's not some mistake with your experiment you can rule out) then that may be evidence for a breakdown of the superposition principle at high energy.
There are currently theories known as spontaneous collapse theories which postulate that, under certain physical circumstances, the superposition principle breaksdown and the wavefunction "spontaneously" or stochastically collapses. The theories vary on what the exact physical circumstances are that induce the collapse. It's possible some of these theories may consider an energy dependency of the collapse.
A: The reason for the decay of superposition at high energy is the instantaneous decomposition of any interaction between particles by high-energy photons. Any preparation of entangled states is immediately destroyed. This is also the reason why we need such low temperatures for a Bose-Einstein condensate.
The only way to achieve superpositions at high energies is to have externally acting fields that are strong enough to keep the entangled particles in their mutual alignment.
